1. Field of the Invention
The present invention relates to an exhaust gas purifying device for a diesel engine, and in particular, it relates to an exhaust gas purifying device by which the rate of purification of the exhaust gas is improved the an ability to provide a good maintenance is provided.
2. Description of the Related Arts
Catalysts are frequently used in known exhaust gas purifying devices used in an internal combustion engines to treat the exhaust gas before it is released to the atmosphere. One of these known exhaust gas purifying devices uses pellets to carry the catalyst, and the pellets are contained in a casing. For example, as shown in FIG. 5 of the attached drawings, a known exhaust gas purifying device comprises a double walled casing 2 with a centrally extending inner tube member 3 connected to an exhaust pipe 1, and catalyst-carrying pellets 4 are contained in the casing 2 outside of the inner tube member 3. The inner tube member 3 is provided with holes 6 to allow a flow of the exhaust gas from the engine through the exhaust pipe 1 and the inner tube member 3 into the area containing the catalyst-carrying pellets 4 in the casing 2, whereby toxic components in the exhaust gas, such as hydrocarbon and formaldehyde, are purified. During maintenance, the catalyst carrying pellets 4 are removed from the casing 2 through a filling opening sealed by a releasable plug 7, and the removed pellets 4 are heated in a furnace to burn away the deposited hydrocarbon and thus regenerate the pellets 4, which can be then refilled in the casing 2.
Other known exhaust gas purifying devices use a honeycomb structure to carry the catalyst, as shown in Japanese Patent Publication No. 61-286513. The honeycomb structure is formed from a porous ceramic filter and comprises a plurality of cells extending longitudinally of the honeycomb. In this prior art, one of the opposite ends of each cell is closed by a plug, and the closed end of one of the cells locates itself downstream of the flow of the exhaust gas and the closed end of an adjacent cell locates itself upstream thereof. Accordingly, the exhaust gas flows into one cell with a downstream cap, penetrates the intervening porous wall between the cells, and flows out from the adjacent cell, whereby the intervening porous walling gradually clogged with particulates in the exhaust gas. The clogged filter is regenerated by burning by a burner or an electric heater.
Nevertheless, several problems arise in the above described prior art devices. Namely, in the case of the exhaust gas purifying devices using catalyst-carrying pellets, the rate of purification of the toxic components is lowered because the particulates are deposited over the surfaces of the pellets. Preferably a large amount of pellets is provided, but this is difficult from the viewpoint of the available space in the automobile. Also, the deposited particles may corrode the exhaust gas purifying device, and further the maintenance of the catalyst-carrying pellets is difficult, in that the catalyst-carrying pellets must be regenerated by a heating treatment at a high temperature such as 700.degree. C., and during this treatment, the pellets are often sintered, and thus permanently deteriorated.
In the case of the exhaust gas purifying devices using the catalyst-carrying honeycomb ceramic filter, a large amount of hydrocarbons is often deposited near the inlet end of the filter, and the burning treatment through the porous walls becomes difficult, and thus the maintenance thereof becomes difficult.